Dic: at (trying to do something, particular condition or state)

17. v PREP n △You use at to show that someone is doing something repeatedly.
6. used after a verb △to show that someone tries to do something, or partly does something, but does not succeed or complete it.

• She lowered the handkerchief which she had kept dabbing at her eyes...
• Miss Melville took a cookie and nibbled at it.
• He clutched wildly at the rope as he fell.
• She nibbled at a sandwich (= ate only small bits of it).

14. v-link PREP n △You use at to say that someone or something is in a particular state or condition.
8. used to show △the situation someone or something is in, what someone is doing, or what is happening

• I am afraid we are not at liberty to disclose that information...
• Their countries had been at war for nearly six weeks.
• The country is now at war.
• I felt like I was at a disadvantage.
• I think Mr. Harris is at lunch.

...... COBUILD, OALD

CF.
16 [PREP] △You use at to say how something is being done.

• Three people were killed by shots fired at random from a minibus...
• Mr Martin was taken out of his car at gunpoint.

19 [PREP] adj PREP n, n PREP n, v PREP n △You use at to indicate what someone is reacting to.

• Eleanor was annoyed at having had to wait so long for him...
• The British team did not disguise their delight at their success...

.... COBUILD

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예문:

• It is not possible to make profits by acting on old information or at patterns of past price changes.

 Let's try to catch different images between:

• act on old information
• act at patterns of past price changes